In some types of equipment, it is necessary or desirable to remove moisture from air or other gas being drawn into the equipment. Examples are large electrical transformers and other high power electrical equipment, such as load tap changers, circuit breakers and conservators, which are immersed in oil to provide electrical insulation and cooling Because the oil expands and contracts with temperature changes, such equipment is typically provided with a head space above the oil or an oil reservoir which is vented to the atmosphere. To inhibit drawing in moisture from atmospheric air which could contaminate the oil, the air drawn in through the vent is passed through a desiccant before entering the tank or reservoir. A typical desiccant in common use is silica gel held in a container through which the air drawn in (and expelled out) passes.
To minimize contact of moisture-laden air with the desiccant when air is not being drawn in, a seal or valve of some sort is usually provided between the desiccant container and the vent entrance. Typical valves include a simple oil cup and mechanical structures that respond to air pressure differentials between the exterior and interior of the container.
Some dehydrating breathers are prone to the phenomena of "channeling" in which air flows primarily over only a part of the desiccant, reducing the effectiveness of the desiccant. Proper design of the breather is necessary to reduce or eliminate channeling.
Over time, the desiccant in such breathers inevitably becomes saturated with water and no longer is capable of adequately drying the incoming air. Consequently, the desiccant material must be periodically replaced or reactivated. Although the replacement of the desiccant is neither costly nor time consuming, this maintenance task is often overlooked. As a consequence, very commonly the gas space over the oil will contain excessive moisture.